Polymers, like polypropylene, are increasingly used in different demanding applications. At the same time there is a continuous search for tailored polymers which meet the requirements of these applications. The demands can be challenging, since many polymer properties are directly or indirectly interrelated, i.e. improving a specific property can only be accomplished on the expense of another property. Stiffness can for instance be improved by increasing the crystallinity and/or the relative amount of homopolymer within the composition. As a consequence, the material becomes more brittle, thereby resulting in poor impact properties. Moreover, the shrinkage of moulded or extruded articles increases with increase crystallinity. It is known that impact strength of polypropylene can be improved by dispersing a rubber phase within the polymer matrix, thereby obtaining a heterophasic polypropylene composition.
Such heterophasic propylene copolymers comprise a matrix being either a propylene homopolymer or a random propylene copolymer in which an amorphous phase, which contains a propylene copolymer rubber (elastomer), is dispersed. Thus the polypropylene matrix contains (finely) dispersed inclusions not being part of the matrix and said inclusions contain the elastomer. The term inclusion indicates that the matrix and the inclusion form different phases within the heterophasic propylene copolymer, said inclusions are for instance visible by high resolution microscopy, like electron microscopy or scanning force microscopy or atomic force microscopy, or by dynamic mechanical thermal analysis (DMTA). Further the heterophasic polypropylene may contain to some extent a crystalline polyethylene, which is a by-reaction product obtained by the preparation of the heterophasic propylene copolymer. Such crystalline polyethylene is present as inclusion of the amorphous phase due to thermodynamic reasons.
One suitable application of such heterophasic polypropylene compositions is its use in tubes such as the field of transmission elements, in particular coated optical fibres, used in telecommunication cables which are typically protected, either individually or as a group, by buffer tubes.
For instance, one or more optical fibres, a group, bundle or ribbon of optical fibres may be protected by a polymeric material in the form of a tube or of a flexible sheath, i.e. the buffertube. The optical fibre together with its protective element is generally referred to in the art as “optical unit”. An optical cable may contain a single optical unit or a plurality of optical units. Said single or plurality of optical units is generally referred to as the optical core of the cable. The optical core is in turn typically inserted into a protecting polymeric jacket. Such a construction is referred to as optical fibre cable.
EP patent application no. 1 024 382 discloses a telecommunication cable comprising a flexible buffer tube made from a thermoplastic polyolefin elastomer having a modulus of elasticity below 500 MPa at room temperature and a modulus of elasticity below 1500 MPa at −40° C. Examples of suitable elastomers are ethylene-propylene copolymers, preferably with more than 10 percent of ethylene monomer, terpolymers containing propylene-ethylene, ultra-low density polyethylene or ethylene-octene copolymers, preferably containing more than 10% by weight of octene monomer. The elastomer can also contain inorganic fillers for controlling physical parameters, such as mechanical properties and flame retardancy.
EP 1 448 705 discloses a heterophasic olefin copolymer having at least one amorphous phase and at least two different crystalline phases. This makes it possible to manufacture tubular elements for telecommunication cables which have improved tearableness. The extrusion will be complicated.
One disadvantage of polypropylene resins is that in most extrusion processes they are subject to significant post-extrusion shrinkage. This means that in applications where shrinkage resp. dimensional stability is important the extrusion parameters must be tailored to the specific composition and the specific extrusion operation to yield a finished part of the precise dimension which is required. This shrinkage problem is particularly troublesome where the manufacturer has several extrusion dimensions and extrusion operations and subsequently wishes to substitute a different composition or halter the process to e.g. increase the cooling rate. This problem is enhanced because polypropylene resins show a post-moulding shrinkage difference in the longitudinal and the transverse direction with respect to the flow direction in processing.
Although a lot of development work has been done in the field of heterophasic polypropylene compositions, it was up to now not possible to find a well-balanced polymer composition with respect to a good balance between stiffness and impact strength in combination with low shrinkage.
Along the present description and claims, the term “tube” is intended to include within its meaning any element which has or can be disposed in a tubular form, especially within the cable structure. Examples of such tubular elements are buffer tubes housing at least one transmission element or polymeric sheaths disposed to surround inner portions of a telecommunication cable, e.g. one or more buffer tubes. Said polymeric sheath is preferably in the form of a tube (e.g. extruded about said inner portion).